1. Field of the Invention
The present invention relates to a lens apparatus and a camera system in which the aperture value is automatically adjusted to keep the brightness of the image constant even when the image taking magnification is changed by an extender lens.
2. Description of the Related Art
In television lens apparatus equipped with an extender lens, when the magnification of the extender lens is changed, the F-number changes with the focal length. In some known apparatuses, the aperture value is corrected according to the magnification of the extender lens, whereby the F-number is controlled to be constant irrespective of the magnification of the extender lens.
For example, according to an apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-162692, in view of the fact that changing the magnification of the extender lens from 1 to 2 results in decrease of the light quantity to half, aperture correction is performed so that the aperture or stop is opened to double the light quantity so as not to change the brightness when the magnification of the extender lens is changed.
In connection with this, there is an error or difference between the theoretical aperture value that is optically determined and the aperture value adjusted by electrical control. In the case where the error is constant for all the aperture values, an offset equivalent to the difference may be added to the aperture correction amount.
In the case where the error varies depending on the stop value, if correction is performed with a constant correction value, accurate correction cannot be achieved at aperture values involving large errors, and changing the extender lens causes a change in the brightness of the image. On the other hand, at aperture values involving small errors, accurate correction is achieved, and changing the extender lens does not cause a change in the brightness of the image.